The present invention relates generally to a crystal growing method and apparatus therefor, and more particularly, to a crystal growing method and apparatus for growing single crystals with a uniform axial oxygen concentration in the crystal.
In the growing of crystals for the manufacture of very large-scale integrated circuit devices, it has been discovered that it is essential that the oxygen concentration in the as-grown crystal be controlled as accurately as possible. In this regard, it has been found that oxygen incorporated in the crystal lattice is important for gettering impurities out of the crystal. Additionally, the presence of oxygen in the crystal lattice provides the crystal with a required hardness characteristic. Accordingly, it is highly desirable to produce a semiconductor crystal having a uniform oxygen concentration throughout the length of the crystal.
Axial non-uniformity of the oxygen concentration in the crystal lattice is a particular problem in standard crystal growing techniques such as the Czochralski technique. Referring particularly to the Czochralski technique for growing semiconductor single crystals, a high purity semiconductor material is melted in a container and the temperature of the molten material is maintained just above the melting point of the material. A particularly oriented seed crystal is then dipped into the melt liquid or material adheres to the seed by surface tension and adhesive forces. Under the correct conditions, a crystal will grow as the seed is slowly pulled away from the melt.
With this technique, oxygen enters the melt in the following manner. The crucible in which the melt is contained is typically made either of silica or graphite coated with silica. At the melt temperature of silicon (about 1400.degree. C.), the surface of the silica crucible which is in contact with the melt dissolves and forms silicon monoxide, SiO. This silicon monoxide enters the melt and essentially constitutes the source of oxygen in the melt and in the drawn crystal.
It has been found that the oxygen concentration in the crystal from this silicon monoxide is not constant, but varies from the seed end of the crystal, where it is at its highest level, to the tail end of the crystal, where it is at its lowest level. Initially, the oxygen content of the melt is on the order of 2.times.10.sup.18 atoms per cubic centimeter (approximately the saturation point). The oxygen in the grown crystal pulled from this melt ranges from approximately 1.5.times.10.sup.18 atoms/cc in the seed end of the crystal down to approximately 6.times.10.sup.17 atoms/cc at the tail end of the crystal. It is thus apparent that the oxygen content of the melt is depleted during the crystal growing process. It is speculated that this oxygen depletion in the melt is due to a lower dissolution rate of the crucible as the growing process proceeds and the melt level in the crucible decreases.
The above described oxygen concentration axial non-uniformity in the semiconductor crystal means that a substantial portion of the crystal will be lost or rejected because of the oxygen level being outside specification. Additionally, this axial non-uniformity imposes the requirement that any wafers cut from a given axial position in the crystal boule be analyzed and sorted according to their particular oxygen content. This wafer testing and sorting in accordance with oxygen content is both costly and time consuming, and thus highly undesirable in a production environment.
There have been a number of different approaches to controlling the oxygen content in semiconductor crystals to thereby avoid these testing and sorting steps. By way of example, Hoshikawa et al. Japanese Journal of Applied Physics, Vol. 19, No. 1, January 1980, pp. L33-36, discloses a silicon growth process for obtaining axial uniformity of the oxygen concentration in the crystal by rotating the seed, the crucible, and the melt. The melt is rotated by means of a rotating magnetic field applied via the application of a three-phase current to a three-phase graphite heater. The reference describes the use of various different rotation directions for the seed, the crucible, and the melt. In one particular experiment, the seed, the crucible, and the melt were all rotated in the same direction but the seed rotation rate was linearly decreased from 20 to 10 rpm during the top-to-bottom growth process of the crystal ingot. However, the crystal resulting from this variation of the seed rotation rate did not have a sufficiently uniform axial oxygen concentration for semiconductor manufacturing purposes. Additionally, the variation of the seed rotation rate caused a concomitant loss of uniformity in the radial oxygen concentration in the crystal.
The reference Kim et al., IBM Technical Disclosure Bulletin, Vol. 25, No. 5, October 1982, p. 2277, also is directed to controlling crystal oxygen concentration and specifically discloses a silicon growth process wherein the rotation of the melt is first accelerated, then decelerated, by means of the periodic interchange of two of three phases of a signal applied to a three phase AC heater. However, this periodic acceleration and deceleration of the melt rotation rate is for the purpose of obtaining a uniform mixing of the melt in the radial direction, and is not concerned with the axial oxygen concentration in the crystal.
The invention as claimed is intended to remedy the above described oxygen concentration non-uniformity along the length of the crystal.
The advantages offered by the present invention are that a uniformity of oxygen concentration is obtained axially along the length of the crystal. Likewise, the oxygen segregation/mixing in the crystal is controlled. This resulting uniformity of oxygen concentration along the length of the crystal permits the elimination of the previously required oxygen concentration testing and sorting steps. Furthermore, a significant increase in crystal growth yield is obtained.